This invention relates to a process for the production of a shaped article by injection moulding and to the article prepared by the process.
The invention relates more particularly to the production of shaped articles comprising a core of polythene terephthalate, PETP, covered by a covering layer of some other thermoplastics material such as polybutylene terephthalate, PBTP.
When PETP is injected at its normal transformation temperature, which is higher than its softening point and is, for example, of the order of from 210.degree. to 220.degree. C., into a mould heated to the order of 130.degree. C. (i.e. in conditions obviating thermal shock), it acquires by slow cooling, at a temperature a little below 140.degree. C., a very homogeneous opaque semicrystalline structure. This has outstanding mechanical and physical properties, a high resistance to chemical attack and the ability to withstand stress cracking, notably when it contains up to 50% of mineral fillers.
The structure produced tends to have a very low coefficient of heat expansion--1.8.10.sup.-5 --and is virtually totally devoid of internal stresses, with the result that the articles made from the material have very little tendency to distort in use even at high temperatures.
Also, the remarkable rigidity of the end products (flexure modulus of the order of 16,000 meganewtons/m.sup.2) and the high flexure temperature of the order of 240.degree. C., enable the ordinary thickness of the articles to be reduced, with a consequent weight saving.
Also PETP is cheaper than other such crystalline products.
However, despite its qualities when treated under these temperature conditions, PETP has not become widely established. The high filler content which can be used and which contributes in no small way to its excellent properties tends to impair the surface of the end products, which have a shiny and heterogeneous appearance making them impossible to use for purposes where a smooth surface texture is required.
Furthermore, the need to heat the mould, and then to cool down slowly from the injection temperature to less than 140.degree. C. in order to produce a semicrystalline structure, results in considerable energy expenditure and lower rates of output which are generally unsuitable for industrial operations. Also the rigidification temperature of PETP is well below 140.degree. C. and a further factor which slows down output is that, even though the crystallization temperature may have been reached, the article cannot be removed from the mould until it has cooled to the rigidification temperature.